Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. J-G. Zhu et al. has described another spintronic device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. The STO comprises a stack including a spin injection layer (SIL) with PMA character, an oscillating field generation layer (FGL) with in-plane anisotropy, and a spacer between the SIL and FGL.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers typically referred to as a reference layer and free layer are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode in a MRAM device. In another aspect, a MTJ element in a read head sensor may be based on a giant magnetoresistance (GMR) effect that relates to a spin valve structure where a reference layer and free layer are separated by a metal spacer. In sensor structures, the MTJ is formed between two shields and there is a hard bias layer adjacent to the MTJ element to provide longitudinal biasing for stabilizing the free layer magnetization.
In a MTJ within a MRAM or STT-MRAM, a reference layer will usually exert a stray magnetic field upon the free layer that tends to favor either the P or AP state. The stray “offset” field (Ho) has a form similar to a non-uniform electric “fringing” field at the edges of a parallel plate capacitor. As depicted in FIG. 1, the stray field (Ho) 4 from reference layer 1 impinges on the free layer 3. Note that a dielectric spacer 2 such as a tunnel barrier layer separates the free layer and reference layer. When reference layer 1 is a composite, the net stray field 4 will be the sum of fringing fields from several similar layers in the reference layer stack with the possible addition of a uniform effective “interlayer” coupling field. The free layer is subject to random thermal agitation and the stray field will create a disparity in the thermal stability of the two states, with either the P or AP state rendered more thermally stable. This asymmetry is undesirable since for a given free layer coercivity (Hc), Ho should be zero for optimum stability. Generally, Ho=0 is difficult to achieve in practice, and as a rule, Ho<15% of He is a reasonable target in actual devices.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and spintronic devices.
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials which utilize shape anisotropy. Thus, magnetic devices with PMA are advantageous for achieving low switching current and high thermal stability. For spin torque applications, a free layer with high Hc and low offset field (Ho) is required. In addition, the free layer preferably has a high energy barrier Eb=KuV/KBT where Ku is the magnetic anisotropy, V is the switching magnetic volume, KB is the Boltzmann constant, and T is the measurement temperature. A SAF free layer has been employed with a coupling layer (spacer) formed between two ferromagnetic layers (FL1 and FL2) having PMA in opposite directions in order to reduce coupling between a free layer and reference layer in a MTJ stack. Several PMA material systems for FL1 and FL2 include various ordered (i.e. L10) alloys, unordered alloys, and laminates represented by (Pt/Fe)n, (Pd/Co)n, (Ni/Co)n, and the like, where n is the lamination number. Magnetization direction for FL1 and FL2 is anti-parallel due to the RKKY coupling through a metal spacer. There is a big challenge to increase the RKKY (anti-ferromagnetic) coupling strength to enhance magnetic stability and thermal stability of the free layer to be compatible with semiconductor processes that reach as high as 400° C. or higher. A higher annealing temperature of >350° C. is also useful in achieving an enhanced TMR ratio.
None of existing technology is known to provide low Ho with high Hc and a Kb value approaching 70 for thermal stability in a PMA layer that will withstand high temperature processing up to 400° C. or greater which is required in fabrication methods. Therefore, a low cost multilayer stack with high PMA, high Hc, low Ho, and improved thermal stability is needed to enable PMA materials to be more widely accepted in a variety of magnetic device applications.